A fully automatic coffee machine designates devices for obtaining hot beverages, in which at the initiation of an operator a coffee beverage is automatically prepared and dispensed in a sequentially controlled manner. As a rule such fully automatic coffee machines are equipped with a drinking water supply in the form of a water container or a drinking water supply connection, a grinder unit for grinding coffee beans, a brewing unit for preparing the hot beverage from the ground coffee beans and the hot water, and a hot beverage dispensing device in the form of a nozzle or similar. In order to enable different types of coffee drinks to be dispensed, in particular to allow the user to heat up and/or froth up milk with the aid of the fully automatic coffee machine, conventional fully automatic coffee machines are known which additionally enable a fully automatically controlled or also manually controlled dispensation of steam at a steam output nozzle or the like. In order to save components and thereby material, it is usual to equip such a fully automatic coffee machine with only one primary heating arrangement in the form of a continuous-flow water heater. This single continuous-flow water heater serves—under appropriate control by a machine control circuit that is often present—both to prepare hot water for the coffee brewing process (in the range around 100° C.) and to prepare useful steam for output at the steam dispensing nozzle, wherein for this purpose it must heat up the supplied drinking water to temperatures of approximately 160° C. to transfer it into the gaseous condition.
In the case of conventional fully automatic coffee machines that in order to provide the control of the continuous-flow water heater based on demand, it is known to connect a controllable circuit element between the AC voltage supply and the continuous-flow water heater. FIG. 2 shows such a conventional circuit design schematically in the form of a block diagram. An AC voltage (as a rule typical mains AC voltage of 230 V and 50 Hz) fed by an external AC voltage supply 160 is applied to the one power connection of a Triac 145; at the output side the Triac is connected to a fluid heating arrangement, here in the form of a thermoblock 150. To control the Triac, a microcontroller 126 is provided which drives the Triac in accordance with the signal of a fluid temperature sensor 125 that measures the temperature of the fluid at the output of the thermoblock 150. In the conventional control electronics this type of control is only possible in a manner whereby the Triac 145 switches the full AC voltage through to the thermoblock 150 or not over a period from several seconds up to a few minutes.
Conventional fully automatic coffee machines typically consume a power of around 1.4 kW. When the fully automatic coffee machine is turned on for the first time, i.e. directly after being placed into operation, a heating process then takes place by appropriate control using the microcontroller 126, in order to heat up the fluid at the thermoblock to its working temperature, i.e. approximately 100° C. If steam is then to be dispensed, the fluid supplied must be heated up from 100° C. to approximately 160° C. During such a heating-up period TH, which is shown schematically in the diagrams of FIGS. 3a, 3b and 3c, the microcontroller 126 drives the Triac 145 continuously in such a manner that it switches the AC voltage from the AC voltage supply 160 through to the thermoblock 150, until the feedback signal from the temperature sensor 125 indicates that the target temperature of, for example, 160° C. has been reached or exceeded. During the subsequent dispensation of steam, the thermoblock 150 is then turned off at intervals or turned on again temporarily for short temporary intervals TZ of a few seconds, in order to maintain the target temperature for the useful steam of approximately 160° C.
The thermal power PTH (in fact pure effective power due to the almost completely resistive consumer) shown in FIG. 3c follows in this case the effective voltage Ueff at the output of the Triac 145, shown in FIG. 3a. A curve of the fluid temperature θ is then obtained, as is shown in FIG. 3b, for example.
Conventional hot fluid preparation units of this type then have the disadvantage that the time taken for the initial heating, i.e. the period TH is relatively long, for example longer than 5 seconds. If steam dispensation is desired, the system must wait for the entire period TH, which reduces the operational convenience and the maximal quantity of coffee that can be prepared per unit time.